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MG Midget and Sprite Technical - engine missing

Hi, hope someone can help. I took my 76 midget out for a proper run last Sunday for the first time since I got it (as a box of bits) the only other run was back and forth to its MOT. It ran great to start with but after about 10 miles it started to miss underload by the time we got back it was missing all the time and backfiring. I wondered if the timing had slipped but it hadn't. i tried pulling the plug leads off and pulling 3 and 4 doesn't make too much difference to the engine speed but what I did notice when I hold the lead just off the head of the plug the engine speed picks up. Does this make sense and does it indicate what the problem might be?
Thanks.


phil09

Have you tried looking under the bonnet?




I'll get my coat!
Dave O'Neill 2

Is it points ignition, if so have you checked the points again to make sure the gap hasn't closed up. 10 miles of use isn't much time for them to wear but sometimes they do wear rapidly until they bed in.
David Billington

It's possible that the spark is tracking between 3 & 4 - possibly a hairline crack in the distributor cap.

Dave O'Neill 2

yeah thanks Dave I did look under the bonnet I also looked in the boot and the spare wheel is poperly inflated so its not that. I will check the points tomorrow though thanks David
phil09

dave I have replaced the leads the dizzy cap the rotor arm points and condensor. I went out some weeks ago and started it in the dark to check that there wasn't a lightshow happening under the bonnet everything looked fine. What really confuses me is that the engine runs better when the plug lead is not in contact.
phil09

check the plug gaps first

If the spark improves when the lead is off it a bit it may mean the strength of the spark is being increased by it having to jump a little further which may mean the coil itself is getting dodgy

There are far better electricians around here that can no doubt explain what causes this, but I have seen it happen before
Bill

maybe a sticking valve...most likely one of the exhaust

>>>>What really confuses me is that the engine runs better when the plug lead is not in contact<<<<

Does that imply that the engine is running THAT BAD...

prop
Prop

My money is on the rotor or dissy cap.
Bob T

Phil
Bill&Bob are on the money. By introducing a spark gap by pulling the leed back a bit increases the power of the spark and might be too much for the insulation in your plugs. Try swapping the plugs over with nos.1&2 and see if the misfire follows,if it does it is a sparkplug problem. If not have a real good look inside the dist. cap for signs of spark tracking between 3&4 posts . I would think it would be unusual to be the rotor button causing the engine to missfire on only 2 cylinders but I guess anything is possible. Also a compression test while the plugs are out as you will get similar symptoms from a blown headgasket between 3&4 Hope this is of some help Gary
Gary

sorry Correction- Too much power for the plug,s insulation with the leed on NOT off but by pulling the leed back a bit does in fact increase the power of the spark but will allow a plug with dodgey insulation to fire.
Gary

hey gary,

Can that be seen in a dark garage....thanks for the info. I wasnt aware of this phonamonon....is that a re-sult of cheap plugs, manufacture defect, over heating the plugs, rough handling what causes it ????


>>>>>>>>>>by pulling the leed back a bit does in fact increase the power of the spark <<<<<<<<<<<<


I dont understand how this action can produce that result...

prop
Prop

Prop
no you can't see it in the dark.it is internal. I believe it is caused by the fact that some brands of plug are marginal in their insulation and work quite ok with carbon leeds which they are probably designed for but when they are used with solid wire leeds it's a bit much for them Gary
Gary

Prop
It's just like having a cracked insulator in the plug and pulling the leed back off the plug and making it jump a gap to the plug will be enough to let it work semi properly.
On the other subject of the spark being stronger if there is a spark gap introduced into say the coil leed, I have never been able to get my head around what does that but I have had it demonstrated to me on a plug tester and there is definatley a better spark with an airgap in the leed I can remember years ago a bloke used to go around the shows here selling these bakelite airgap thingos that plugged into the centre of your dist. cap and then the coil leed plugged into it. The claim was better power smoother running and better fuel economy. I bet they played a fine tune on the radio as well. Gary
Gary

hello,
i had a problem like that where one of my high tension leads was breaking down with electrolysis, and a screw holding it to the distributor was rusty.
Then i found my fuel filter and an SU float needle was blocked. The midgie was suffocating
GLA72

It would idle but had no power
GLA72

Thanks everyone I swapped the leads around but the problem stayed on the same cylinders. So I swaped the plugs around and the problem shifted to the new cylinders so it was the plugs. I have nnow replaced the plugs and its running well. thanks again
phil09

hey gary,

Agian Thanks you, for posting that bit of new "Odd" insight into damaged plugs,

Ill have to write that one down and remember that the next time I come across a "miss" where pulling on the plug leed makes (or even off and letting it ark across the air gap) it run better as a result due to interior plug problems...not seen in the dark.


learn something new everyday on this sight

prop
Prop

This thread was discussed between 19/07/2008 and 20/07/2008

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